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Old May 4th, 2011, 09:26 PM   #1
iahi_2005@yahoo.com
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Question 4-20 Loop 2 Wire vs 3 Wire

Guys,
I am really confused about 4-20 ma current loop 2 wire vs 3 wire...what is the difference?

Does the difference have anything to do with the sensor being powered separately or in loop?

In My case the sensor is a Gas detector and sends output to PLC Analog Input.So is the sensor being powered from the PLC Analog Input Card?

Thanks..i would really appreciate it if you could clear that up...
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Old May 4th, 2011, 10:28 PM   #2
Mickey
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A picture is worth a thousand words. (or something like that )

Give us the exact part number of your gas sensor or a link to it's manual.
Attached Images
File Type: jpg analogWiring.jpg (28.7 KB, 577 views)
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Last edited by Mickey; May 4th, 2011 at 10:40 PM.
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Old May 4th, 2011, 11:59 PM   #3
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if it is resistance based it is probably used for compensation, simialr to an rtd
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Old May 5th, 2011, 12:45 AM   #4
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Quote:
if it is resistance based it is probably used for compensation, simialr to an rtd
huh?
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Old May 5th, 2011, 08:13 AM   #5
Tom Jenkins
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Quote:
Originally Posted by iahi_2005@yahoo.com View Post
Guys,
Does the difference have anything to do with the sensor being powered separately or in loop?
You are correct! Two wire loop powered sensors are usually simple devices like pressure or temperature transmitters that don't need much power. They use a diode or resistor to create a voltage drop inside the device to power the circuits.

Three and four wire devices are more complex transmiters that need more power than can be easily taken from the loop. Your gas detector is probably three or four wire, but the manufacturer's manual should identify the wiring.

Mickey's post shows the wiring difference perfectly.
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Old May 5th, 2011, 08:38 AM   #6
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Quote:
Originally Posted by Mickey View Post
A picture is worth a thousand words. (or something like that )

Give us the exact part number of your gas sensor or a link to it's manual.
On your picture, is the power supply the analog input card?...or can the analog input card never be a power supply also?
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Old May 5th, 2011, 09:12 AM   #7
Mickey
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Quote:
Originally Posted by iahi_2005@yahoo.com View Post
On your picture, is the power supply the analog input card?...or can the analog input card never be a power supply also?

That would depend on the card your are using. But most that I am familiar with use a separate power supply.


What analog input card do you have ( complete catalog number)?
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If you want happiness for an hour-take a nap. If you want happiness for a day-go fishing. If you want happiness for a month-getmarried. If you want happiness for a year-inherit a fortune. If you want happiness for a lifetime-help someone else.
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Old May 5th, 2011, 10:44 AM   #8
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Quote:
Originally Posted by Mickey View Post
That would depend on the card your are using. But most that I am familiar with use a separate power supply.


What analog input card do you have ( complete catalog number)?
I am using a 1756-IF16 Card (Allen Bradley)
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Old May 5th, 2011, 11:07 AM   #9
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I am using a 1756-IF16 Card (Allen Bradley)
The card does not supply the power. You will need an external power supply.
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File Type: pdf wiring.pdf (186.9 KB, 179 views)
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If you want happiness for an hour-take a nap. If you want happiness for a day-go fishing. If you want happiness for a month-getmarried. If you want happiness for a year-inherit a fortune. If you want happiness for a lifetime-help someone else.
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Old May 5th, 2011, 02:40 PM   #10
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On the platform i work on all the line of sight and point gas detectors supply 4-20mA to the fire and gas cards ie. they have their own power supply. The catalytic gas detectors are powered from the fire and gas cards and work on resistance.
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Old May 5th, 2011, 02:51 PM   #11
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i have some gas monitiors that are two wire 4-20. Google OI-675. Various gas set ups, are Class 1,Div 1 and work from on 12 & 24 VDC systems. Easy as pie but are in the 1.7k range.....
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Old May 5th, 2011, 02:55 PM   #12
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http://www.bapihvac.com/content/uplo...igurations.pdf
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Old May 5th, 2011, 10:03 PM   #13
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Quote:
Originally Posted by Mickey View Post
A picture is worth a thousand words. (or something like that )

Give us the exact part number of your gas sensor or a link to it's manual.
Looking at the picture the only difference between the 2 wire and 3 wire is the extra ground wire...but what is the significance of that?...
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Old May 5th, 2011, 10:29 PM   #14
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Quote:
Looking at the picture the only difference between the 2 wire and 3 wire is the extra ground wire...but what is the significance of that?...
Read Tom's second paragraph post #5.

Look again at the drawings. The 2 wire device is loop powered. The 3 wire device is first powered by the 24vdc power supply and has a separate output which goes to the PLC's input, it's not loop powered. The power supplies common and the PLC's minus input are tied together to complete the 3 device's output circuit.
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Old May 5th, 2011, 11:17 PM   #15
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Quote:
Originally Posted by Mickey View Post
Read Tom's second paragraph post #5.

Look again at the drawings. The 2 wire device is loop powered. The 3 wire device is first powered by the 24vdc power supply and has a separate output which goes to the PLC's input, it's not loop powered. The power supplies common and the PLC's minus input are tied together to complete the 3 device's output circuit.
I am sorry to keep asking..if like you say that is the case what is the difference between 3 wire and 4 wire they are both separetely powered.....when would you use one over the other
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